Full Idea
A lunar eclipse always has a convex dividing line, so, if it is eclipsed by the interposition of the earth, the circumference of the earth, being spherical, is responsible for the shape.
Gist of Idea
The Earth must be spherical, because it casts a convex shadow on the moon
Source
Aristotle (On the Heavens [c.336 BCE], 297b29)
Book Reference
Aristotle: 'On the Heavens I and II', ed/tr. Leggatt,Stuart [Aris and Phillips 1995], p.167